1. Field of the Invention
This invention relates to vessels for holding a liquid therein that are interchangeable within a larger apparatus and where the liquid level therein must be monitored.
2. Description of the Prior Art
There are many modern apparatus that utilize liquid contained within an interchangeable vessel in their function. One example would be a modern computer printer. These devices typically include a disposable ink cartridge that contains the ink used to perform the printing operation. It is advantageous if it could be determined when the level of these cartridges is getting low. With typical black ink, the ink contains sufficient carbon that it is known to use an electrical current to determine whether or not a certain level of fluid remains within a cartridge. A problem has arisen with respect to the new colour printers in that the coloured ink used therein does not have sufficient carbon to provide an adequate electrical signal path. Mechanical apparatus is also known to measure liquid level and typically involves a float coupled to a mechanical switch or potentiometer connected thereto. A problem with these devices is that they must be in physical contact with the fluid within the cartridge and therefore take up space therein. In addition, mechanical structure such as this would be relatively expensive even in the high qualities that are envisioned for interchangeable cartridges such as these. It would be difficult for a user to recycle the mechanical apparatus due to the associated mess involved in interchanging the devices between the old and replacement cartridges. It is also known to use ultrasonic measuring of liquid levels by measuring the time required for an acoustic signal to travel across a portion of the cartridge where the ink would be disposed. As such systems have their moving parts, they have good reliability; however, most are relatively complex and expensive which is a major drawback when dealing with a vessel that must interchangeable and may in fact be disposable.
It is further known by way of U.K. Patent Application No. 9513267.6 entitled "Liquid Level Switch" to incorporate the benefits of acoustic liquid level measuring without some of the associated problems. In this application a sensor that is fixable upon an outer surface of a tank wall is disclosed. The operating concept involves sending an acoustic pulse through the tank wall and detecting the characteristics of a reflection that would be caused by the tank wall air interface versus the tank wall liquid interface. Therefore, it is desired to incorporate this technology into an interchangeable vessel that would be easy to use and economical in form.